From a paper on strange parasitic relationships, this story of a shark with malice on his mind and eels in his heart. Actually the shark was caught in a fishing competition, so there probably wasn’t any malice, just eels.
This was a facultatively parasitic relationship. In other words, the eels didn’t need to be living in the sharks heart (that would be obligate parasitism), rather they took advantage of an opportunity to get a meal. They proposed that the eels probably attacked the shark after it had been hooked and was dangling, distressed, from the longline. They had some evidence that the shark was probably resting on the bottom, which may have made it easier for the eels to find. The pugnoses somehow gained entry (hypothesised to be through the gills) and made their way to the heart, where they dined on the beasts blood up until it died. Maybe they would have burrowed out again after the animal expired, maybe they would have suffocated (remember – the eels had be swimming in and breathing the sharks blood once they were inside, how bizarre is that?).